Hot Water Heat Pump

For most folks, the second most energy intensive activity in the home (after living space heating/cooling) is heating potable water. For a great many people the obvious choice is storage-based or on-demand fired by natural gas. But lots of folks (like me) don’t have natural gas service so we usually rely on storage-based electric water heaters. They’re relatively inexpensive to purchase (maybe $300-$350 or so for a halfway decent 50 gallon unit) but expensive to operate. Standard government estimates run around $500-$550 per year. This figure depends a lot on your usage and local electricity rates.

By themselves, electric resistive water heaters are relatively efficient in simple terms. Generally, between 90 and 95 percent of the electrical input is translated to heating water. This, of course, does not account for generation and transmission of said electricity, and as the average consumer is many miles from a generation plant, the system efficiency is much, much lower. In other words, bringing the fuel to the consumer (e.g. natural gas) and having them burn it on site achieves a much higher system efficiency.

Ultimately, an electric water heater is not much different from a toaster or space heater: You pass current through a resistive element, the element heats up, which in turn heats the water (or the bread, or the air).  So how do you make a system like this more efficient and less costly to operate?

Drastically decreasing energy consumption might seem like an insurmountable problem given that these units are already hitting 95 percent efficiency. But there is a way, and that way is to use the electrical input to move heat rather than create heat. That’s what a heat pump does. It moves heat from one place to another. The source of that heat could be the air or the ground or even a large pond of water. And the source doesn’t have to be all that “hot” to begin with. There’s actually quite a lot of heat in a pond of 50 degree Fahrenheit water.  Think of it this way: Suppose you had a gallon of diesel fuel. You could burn it directly to heat your house. Or, you could put it in your truck, drive to a wooded area, gather up some standing dead wood, drive it back to your house, and burn that. You could get a lot more BTUs through the wood than is in a gallon of fuel. In this situation we’re interested in an efficiency factor. That is, how much energy we “purchased” or derived for a given energy input. The efficiency factor for a basic heat pump can be around 3 for even a modest air-source heat pump.

So, what we’ll do is use a heat pump for water heating. Much more efficient! Unfortunately, unless you want a large heat pump, the recovery rate isn’t going to be as good as a standard electric unit. OK, so we can combine the two, or at the very least, use the heat pump as a pre-heater. This could be easily attached to an existing hot water tank, and in fact, a few companies in the USA offer them, such as Geyser and E-Tech. (My understanding is that while this idea is relatively new to the US mainstream and up to now has been a niche market, it’s been in use in Europe for some time.)

Recently, all-in-one units have been offered by Rheem and GE. The integration of the heat pump and hot water tank offers some advantages in terms of design. The main disadvantage of these units, whether integrated or add-on, has always been up-front cost. Add-ons are in the $1k range and integrated units considerably more. With recent energy and tax incentives, though, the price is not a major issue.

Our 18 year old 40 gallon electric unit began to go this winter. There were some tell-tale signs of leaks beginning. What to do? After investigating the options and incentives, I decided to go with the integrated GE unit. I purchased this through a local home improvement store. The going price is $1600. Yeow! Yep, that’s at least $1200 more than a so-called “high efficiency” normal electric model. The government’s energy rating places it at about $200 per year, or less than 40% of the operational cost of normal models. OK, so the payback for that is not quick, but certainly doable. Granted, it’s only the two of us and we probably don’t use as much hot water as the average family (although our electricity rates are probably higher), but it’s still advantageous.

Wait. It gets better. These units fall under the 2009 energy act and you can get a tax credit of 30% of the installed cost (not just the cost of the unit itself). It also turns out that our electricity supplier, National Grid, offers an efficiency rebate of $400 for heat pump water heaters. When you combine these, the installed cost differential was only about $250, and we should make that back in about  a year.

But wait, there’s even more! This air-source heat pump is located in our basement. As such, it pulls heat out of the basement air and also dehumidifies it. This is perfect for the summer: Free cooling and dehumidification.

OK, so how well does it work? It’s been installed about a month. The footprint is pretty much the same as the unit it replaced, although it reminds me vaguely of the sort of robot you might see on Futurama. There is a fan on the unit. I’d say it’s about as loud as one of those little hand-held vacuum cleaners, but not as high pitched and whiny. Definitely louder than a refrigerator but way quieter than our furnace. In the winter the basement is rather dry so there hasn’t been any condensate. I was a little concerned that it would overly cool the basement, but at most, it seems to depress the temperature by 2 or 3 degrees Fahrenheit, and for only a couple of hours. Let me explain further.  If someone takes a shower, the unit will run for about 2 hours to recharge. After about an hour, the basement temperature will drop from its normal winter level of 61 F to about 59 F. Once the unit shuts off, the temperature will rebound back to 61 very quickly. I assume that this is due to the fact that the basement is essentially connected to this huge heat source called The Earth on three sides and floor (we have a walk-out so the fourth wall is studded). During the summer, the basement will rise to the upper 60s (maybe 72 after many days of 85+ degree weather). Consequently, I don’t expect to see much in the way of cooling in the summer, but I wouldn’t mind a couple degrees if I can get it, especially as it’s free. We’ll have to see about the dehumidification as well. It gets amazingly humid down there in the summer, and if we can back off on the use of the stand-alone dehumidifier, that would be a plus.

I’ve only received one energy bill since installation, and only one of the weeks was covered by the new unit. The bill was a few dollars lower than expected, but within the margin of error. I intend to track it over the next few months to see just how much energy savings we derive.

So far, so good. And here’s a link for some good comparative analysis:

Update, April 13, 2010: We’ve just received our March electricity bill and we went from an average usage of about 14 KWH/day in 2009 to about 11 KWH/day. If we continue with this percentage, we should meet the projected recovery of the initial investment within a year. Also, now that the weather has warmed a bit and the humidity has increased, we’re starting to extract some water from the basement air. I have little doubt that the summer dehumidification will scale up considerably.

Author: jim

Jim is a college professor with a fondness for running shoes and drumsticks.

One thought on “Hot Water Heat Pump”

  1. Heat source pumps are great and ours has reduced space heating and water heating costs to such an extent that our winter bills fell by one third for 2009-2010. But, all the talk of pay back time is misleading. After all, we need heating and hot water like we need a fridge or a car for modern living. We don’t look for payback on those items, so why do we apply such rules to energy efficient devices?

    The whole point is to save energy, lower the carbon foot print of our homes and to be more environmentally responsible. If your unit pays back over, say 7 or 10 years or whatever, that’s a bonus.

    So good luck with your unit.

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